This invention relates to an ultrasonic diagnosis device. More particularly, this invention relates to a technology that will be useful for digitization of a signal processing that acquires tomograms having high image quality irrespective of an object and an imaging portion, in an apparatus for non-destructively inspecting an object by ultrasonic waves or in an ultrasonic diagnosis device for conducting medical diagnosis.
A method called a xe2x80x9cfrequency shift beamforming methodxe2x80x9d is known as one of the signal processing methods for forming ultrasonic tomograms by using reception signals from ultrasonic transducer elements that are disposed in an array. This method comprises the steps of mixing the signal from each ultrasonic element with a reference signal, extracting a low frequency component of each mixed signal, or in other words, moving it towards the low frequency side, delaying the mixed signals by respective delay circuits to beamforming, then adding the signals, and obtaining ultrasonic response signals focused to desired positions. References associated with the present invention include U.S. Pat. No. 4,140,022, JP-A-52-20857 and U.S. Pat. No. 4,983,970.
JP-A-6-313764 discloses the technology that is based on the frequency shift beamforming method described above and the major proportion of the signal processing circuit is a digital circuit. This technology employs particularly an over-sampling technique that executes an accumulation processing after each sampled ultrasonic signal is mixed with a reference signal, and improves effective accuracy of analog-to-digital conversion.
On the other hand, JP-A-9-206298 discloses an ultrasonic diagnosis device employing digital orthogonal wave detection. In this reference, a signal from each ultrasonic transducer element is delayed by a delay circuit. Next, the delayed signal is inputted to an orthogonal detection circuit, and the in-phase component and the orthogonal component are generated. This orthogonal detection circuit executes interpolation that is equivalent to changing the frequency of the reference wave to cope with a spectrum shift in which the center frequency of an ultrasonic echo shifts to a low frequency range with the echo time.
It is not easy to drastically change the frequency of the reference wave in a system which executes the effective change of the reference wave by interpolation. In the digital orthogonal detection system of the known reference described above, the high frequency signal generated by digitizing the ultrasonic signal from each element is beamformed by the delaying means before the frequency shifts to the low frequency range. Therefore, high accuracy is necessary for controlling the delay time of each signal channel.
The present invention is based on the premise of the frequency shift beamforming method that exploits the effect of over-sampling. In other words, an ultrasonic signal from each element is sampled at a frequency higher than a Nyquist frequency of the upper limit of its signal band, and is subjected to an accumulation processing in a subsequent stage. The digital signal so sampled is then mixed with a reference signal and is beamformed by a delaying circuit after the accumulation processing is done and then added.
In such a digitized ultrasonic diagnosis device in accordance with the frequency shift beamforming method, it is one of the objects of the present invention to freely vary the frequency of the reference signal and to make it possible to conduct various high-quality or high-speed imaging.
In an ultrasonic diagnosis device comprising primarily digital circuits, it is one of the more concrete objects of the present invention to provide a technology capable of conducting harmonic imaging.
In an ultrasonic diagnosis device comprising primarily digital circuits, it is another object of the present invention to provide a technology capable of simultaneously forming a plurality of beams having different frequencies.
It is a further object of the present invention to improve resolution in a lateral direction and to thus improve quality of ultrasonic images.
The above and other objects and novel features of the present invention will become more apparent from the description of the specification in connection with the accompanying drawings.
A typical construction of an ultrasonic diagnosis device according to the present invention comprises wave transmitting means for repeatedly driving an ultrasonic element group disposed in an array and generating ultrasonic waves; digital converting means for sampling reception signals obtained by the ultrasonic elements at a sampling frequency higher than a Nyquist frequency of the upper limit of the signal band and converting them to digital signals, respectively; mixing means for multiplying the digital signals by a reference signal, respectively; accumulating means for accumulating the multiplied digital signals for plurality of samples, respectively; receive focusing means for imparting a delay to each digital signal so accumulated for aligning the phase difference peculiar to each ultrasonic element and adding the digital signals; and mixing data generating means for supplying serially the reference signal to the mixing means. More concretely, the mixing data generating means includes data computing means for computing in advance a train of reference signals corresponding to the sample point number of the reception signals, and memory means for storing the reference signal line so computed and outputting serially the stored reference signals under read address control corresponding to the sample number of the reception signals.
In general ultrasonic tomograms imaging, the mixing data generating means supplies the signal, the frequency of which decreases gradually from near the center frequency of the transmission wave with the lapse of time, as the reference signal train is provided to the mixing means. In consequence, a mixing processing can be conducted in such a manner as to correspond to a spectrum shift in which the frequency band of the reception ultrasonic waves shifts to the low frequency side as an echo depth increases, and quality of the ultrasonic image can be improved.
Data acquired by digitizing signals having a frequency that is some multiples of the center frequency of the transmission ultrasonic waves are prepared as a reference signal train and are used serially for mixing. In this way, harmonic imaging can be executed easily. In this case, too, image quality can be improved by applying means for gradually decreasing the frequency.
A peculiar construction utilizing the characteristic in which the signal from each ultrasonic element is over-sampled can be employed. Namely, wave transmitting means are provided two times to the construction described above, and two beams having mutually different center frequencies xcfx891 and xcfx892 and different focus directions are synthesized and transmitted simultaneously. On the other hand, the mixing data generating means generates alternately a reference signal generated from a signal having a center frequency near xcfx891 and a reference signal from a signal near xcfx892, and multiplication is effected by using the reference signals that change alternately. The accumulating means and the receive focusing means are disposed two times, too, so that the outputs of the mixing means can be assorted and supplied by the multiplexer. One of the two receive focusing means conducts beamforming in the direction of the beam of xcfx891 and the other, in the direction of the beam of xcfx892. According to this construction, a plurality of ultrasonic beams are synthesized in wave transmission and are transmitted. In the reception signal processing, on the other hand, the beams for applying an appropriate beamforming processing are substantially selected by alternate switching of the reference signals. In this way, high-speed imaging becomes possible.
It is possible to constitute another construction by utilizing the wave transmitting means for synthesizing and transmitting a plurality of ultrasonic beams. The wave transmitting means synthesizes an ultrasonic beam having a center frequency xcfx891 and a first focus in a near distance and an ultrasonic beam having a center frequency xcfx892 which is lower than xcfx891 and a second focus in the same direction as the first focus but in a far distance, and transmits the synthesized signal. The portion of the reception signal processing is exactly the same as ordinary imaging described above. Mixing is executed by the reference signal train acquired from the signal the frequency of which decreases gradually. According to this construction, multi transmit focusing is made by simultaneous wave transmission. Consequently, imaging can be made at a high speed while resolution in the lateral direction is improved in a broad depth range.